Formation of Dense Pore Structure by Te Addition in Bi

1 Department of Energy Science, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Republic of Korea 2Materials R&D Center, Samsung Advanced Institute of Technology, Samsung Electronics, Yongin, Gyeonggi 446-712, Republic of Korea 3 Powder Technology Department, Powder and Ceramics Division, Korea Institute of Materials Science, Changwon, Gyeongnam 642-831, Republic of Korea 4 School of Energy, Materials and Chemical Engineering, Korea University of Technology and Education, Cheonan, Chungnam 330-708, Republic of Korea 5 Centre for Integrated Nanostructure Physics, Institute of Basic Science (IBS), Daejeon 305-701, Republic of Korea


Introduction
Since 1950s, Bi-Te-based thermoelectric (TE) materials have been intensively investigated in order to realize highly efficient power generation from low-grade heat (<250 ∘ C) or solid-state cooling and heating system.However, current applications are restricted to small scale systems such as climate control seat mainly due to their low TE performance.TE conversion efficiency is defined in terms of a dimensionless figure of merit,  =  2 / tot , where  is the electrical conductivity,  is the Seebeck coefficient, and  tot is the total thermal conductivity at a given absolute temperature ().Because  value is controlled by electronic ( and ) and thermal transport properties (), we can obtain the enhanced  by following two approaches.The first approach is to reduce the lattice contribution ( lat ) of  tot by promoting phonon scattering while maintaining the electronic transport properties.The other approach is breaking the tradeoff between  and  through the density of states (DOS) engineering near at Fermi level.
Recently, nanostructuring has shown to be one of the most effective ways to reduce the  lat of Bi 2 Te 3 -based TE materials.Many works of the literature have confirmed that enhanced phonon scattering at the interfaces can effectively reduce the  lat without a significant reduction of  [1][2][3][4][5][6].There are two main approaches to prepare Bi 2 Te 3based nanostructured bulks.The first is the fabrication of nanograined structure to obtain the high density of the grain boundaries to scatter phonons.By formation of nanograined structure, a further reduction of  lat becomes possible by intensified mid-and long-wavelength phonon scattering.
There have been many experimental reports on nanograined structure Bi 2 Te 3 -based TE materials with enhanced  values [1,2].For example, nanograined structure fabricated by high-energy ball milling combined with hot pressing raises the maximum  value of p-type Bi 0.5 Sb 1.5 Te 3 polycrystalline bulk up to 1.4 at 373 K from 1.0 at 300 K for its crystal ingot [1].The second is the formation of nanoinclusion composite.Phase boundaries between TE material matrix and introduced nanoinclusions can act as effective phonon scattering centers; however, nanoinclusions through this approach limit  enhancement up to ∼20% so far mainly due to the difficulty of controlling the size and distribution of nanoinclusions [7][8][9].
Here we report on the newly developed nanostructuring approach for formation of pore structure in Bi-Te-based TE materials.We investigated the relation between the pore structure and TE transport properties.The origin of the enhanced  could be elucidated from the viewpoint of the carrier filtering effect in addition to the phonon scattering in the presence of densely formed pore structure.

Experimental
Bi 0.5 Sb 1.5 Te 3 ingots were prepared by conventional solid-state reaction of high-purity elemental Bi, Sb, and Te (>99.99%,5N Plus).Bi, Sb, and Te granules were weighed with a stoichiometric ratio of the elements and sealed in an evacuated fused silica tube 12 mm in diameter.The tube was heated to 1073 K in a box furnace for 10 h and then quenched in water.The sample was ball-milled in N 2 -filled stainless steel vessel for 10 min using a high-energy mill (8000D, SPEX, USA).Ground Bi 0.5 Sb 1.5 Te 3 powders show a wide grain size distribution ranging from a few nanometers to tens of microns as shown in Figure 1 The Te coated Bi 0.5 Sb 1.5 Te 3 powders were consolidated in a spark plasma sinter (SPS) using graphite die (diameter = 10.5 mm) in dynamic vacuum under an applied uniaxial pressure of 50 MPa at 753 K for 3 min.Then, the consolidated samples were annealed at 640 K for 20 h by the same evaporation method in order to remove the residual Te.The phases of Te coated Bi 0.5 Sb 1.5 Te 3 powders were analyzed by powder X-ray diffraction experiments using an X-ray diffractometer equipped with Cu K radiation ( = 1.5418Å).Seebeck coefficient and electrical conductivity measurements from 300 K to 450 K were performed using an ULVAC ZEM-3 system.The  values ( =     ) were calculated from measurements taken separately: sample density (  ), heat capacity (  ), and thermal diffusivity () measured under vacuum by laser-flash method (TC-9000, ULVAC, Japan), in which   was used as a constant value of 0.186 J g −1 K −1 .

Results and Discussion
In order to fabricate the pore structure in Bi 2 Te 3 -based TE materials, we introduced the Te metal element with high vapor pressure (723 K) onto the surface of Bi 0.5 Sb 1.5 Te 3 powders using evaporation method.Te layer with 8-11 m thickness was coated onto the surface of Bi 0.5 Sb 1.5 Te 3 powders (Figure 1(c)).The Te contents of final Te coated Bi 0.5 Sb 1.5 Te 3 powders did not show significant change with the weight of Te loading (2-80 wt.%) for evaporation and were ranging from 1 to 3 wt.% of Bi 0.5 Sb 1.5 Te 3 powders.This might be related to the nanowire-like growth of Te with restricted length and diameter as shown in Figure 1(d).The growth mechanism of Te nanowires will be published elsewhere.The powder X-ray diffraction (XRD) patterns of Bi 0.5 Sb 1.5 Te 3 and 3 wt.%Te coated Bi 0.5 Sb 1.5 Te 3 powders are shown in Figure 2. All patterns are indexable to the Bi 2 Te 3 as a major phase and small amount of Te was also noticed.Peaks for other phases were not detected in all compositions.From the microstructure and XRD pattern, we concluded that Te coated Bi 0.5 Sb 1.5 Te 3 powders could be successfully prepared by the present evaporation route.The relative densities of SPS compacted bulks of  wt.% Te coated Bi 0.5 Sb 1.5 Te 3 powders (TeBST, ∼95%) did not show significant change compared with SPS compacted bulks of Bi 0.5 Sb 1.5 Te 3 powders (BST, ∼96%).However, both of  and  values of 2TeBST and 3TeBST were lower than those of BST as shown in Figures 3(a) and 3(b).This result suggests that coated Te remained within the bulks and deteriorated the electronic transport properties due to its metallic characteristics.To remove the residual Te, TeBST (inset of Figure 3(b)).This might be considered to be related to the carrier filtering effect at the surface of pores. is related to the energy derivative of the electronic density of state () and the relaxation time () through the Mott relation [10].
Thus  enhancement can be attained by a carrier filtering effect, a strong energy dependence of () caused by band bending at the surface of pores [11].Experimental evidence of an enhancement of  has also been reported in Bi 2 Te 3 -based materials [12].We evaluated the temperature dependence of the  tot of 2TeBSTa and 3TeBSTa.The results are shown in Figure 5(a).Those of the BST are shown for comparison.Over the whole measured temperature range the value of  tot shows significant reduction compared to those of BST.In order to clarify the phonon scattering effect by pores we calculated the  tot using the following equation (inset of Figure 5):  lat =  tot −  ele , where the electronic contribution ( ele ) is estimated from the Wiedemann-Franz law,  ele =  0 , with the Lorentz number  0 = 2 × 10 −8 W Ω K −2 .Extremely low values of  lat ranging from 0.35 to 0.39 W m −1 K −1 at 300 K were obtained in both the 2TeBSTa and 3TeBSTa.In comparison with the value of reference BST ( lat ∼ 0.71 W m −1 K −1 at 300 K),  lat decreased by about 50%, indicating stronger phonon scattering in the presence of densely formed pore structure.
Figure 5(b) presents the s for 2TeBSTa and 3TeBSTa as a function of temperature.A high  above 1.24 at 300 K was realized in 3TeBSTa, and this high  value should be originated from its low  lat value.By formation of pore structure without significantly affecting the charge carriers, this concept is an effective way to improve the TE performance by scattering phonons in addition to enhancement in  by carrier filtering effect.Nevertheless, a novel processing technique for the generation of nanosize pores with <50 nm diameter is highly needed to realize the maximum  by nanostructuring approach-based nanosized pore.Combined technique of nanosized powders fabrication for increasing the Te coating surface and controlled evaporation for forming the nanoscaled Te layer will give the possibility of further improvement of TE performance.

Conclusions
We successfully fabricated the dense pore structure in Bi 0.5 Sb 1.5 Te 3 -based TE materials by evaporation method of Te metal elements.The TE transport properties were investigated in the viewpoint of the pore structure.The characteristic TE properties are summarized as follows.
(1) The thermal conductivity was largely reduced due to the significant phonon scattering by nanosized pores, leading to the extremely low lattice thermal conductivity of 0.35 W m −1 K −1 at 300 K.
(2) The electronic transport properties were not much affected due to the enhancement of Seebeck coefficient, which is presumably attributed to the carrier filtering effect originated from nanosized pores.
(3) The dense pore structured Bi 0.5 Sb 1.5 Te 3 TE materials show the enhanced dimensionless figure of merit,  of 1.24 at 300 K.

Figure 1 :
Figure 1: (a) The SEM image of Bi 0.5 Sb 1.5 Te 3 powders, (b) schematic illustration of evaporation process, and (c, d) SEM images of Te coated Bi 0.5 Sb 1.5 Te 3 by evaporation method.
(a).Then, Te coated Bi 0.5 Sb 1.5 Te 3 powders were fabricated by evaporation method.Figure1(b) illustrates a schematic diagram of evaporation process.Firstly, Te powders (2, 6, 10, 20, 40, 60, and 80 wt.% of Bi 0.5 Sb 1.5 Te 3 powders) were placed at the end zone of quartz tube and Bi 0.5 Sb 1.5 Te 3 powders were loaded into the center part of quartz tube.The dead end part of quartz tube (Te powders) was placed in the muffle furnace with the open end of quartz tube connected with the vacuum pump, and then the furnace was heated to 370 ∘ C for 1-6 h, while the zone for Bi 0.5 Sb 1.5 Te 3 powders keeps maintaining at room temperature.

3 2𝜃Figure 2 :
Figure 2: Powder X-ray diffraction patterns of Te loaded Bi 0.5 Sb 1.5 Te 3 by evaporation method.This clearly shows that the stoichiometric composition of Bi 0.5 Sb 1.5 Te 3 is not changed by the present Te evaporation method.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: The temperature dependences of (a) electrical conductivity and (b) Seebeck coefficient for SPSed Te coated Bi 0.5 Sb 1.5 Te 3 samples and their annealed samples.